Titanium dioxide exists in nature as three different phases namely, anatase, rutile and brookite. It is mainly sourced from ilmenite ore, the most wide spread form of titanium dioxide-bearing ore around the world. Rutile is the next most abundant and contains around 98% titanium dioxide in the ore. The minerals rutile and brookite as well as anatase all have the same chemistry, but they have different structures. Rutile is the more common and well known mineral of the three, while anatase is the rarest. Anatase shares many of the same or nearly the same properties as rutile such as luster, hardness and density. However due to structural differences anatase and rutile differ slightly in crystal habit and more distinctly in cleavage. The metastable anatase and brookite phases convert to rutile upon heating.
Titanium dioxide, particularly in the anatase form, is a photocatalyst under ultraviolet (UV) light. Titania acts as a photosensitizer for photovoltaic cells, and when used as an electrode coating in photoelectrolysis cells, it can enhance the efficiency of electrolytic splitting of water into hydrogen and oxygen.
The photocatalytic activity of titania results in thin coatings of the material exhibiting self-cleaning and disinfecting properties under exposure to UV radiation. These properties make the material a candidate for applications such as medical devices, food preparation surfaces, air conditioning filters, and sanitary ware surfaces. It is also used in dye-sensitized solar cells, which are a type of chemical solar cell (also known as a Gratzel cell).
Titania particles possess large band gap (around 3.2 eV) and as a result, UV light (wavelength<387 nm) is required for its photocatalytic activity. For practical applications, it is imperative to use solar light, however, solar light contains only about 4-5% of UV light. In order to utilize the whole spectrum of solar light, surface modification of titania particles is required so as to make them active in visible range (400-700 nm) as well. The surface of titania particles can be doped with metal and non-metal atoms which extend their absorption spectrum in the visible region and thereby enhance overall photocatalytic activity of the titania particles.
Mechanochemical method for doping TiO2 matrix for large scale production of doped titania nanoparticles is disclosed in the prior art. In mechanochemical method, titania particles are ground with a precursor salt using milling media for a specified time.
Yin et al. in Solid State Ionics 172 (2004) 205-209 reported synthesis of photocatalytic nitrogen doped TiO2 by planetary ball milling of P25 (Degussa) titania powder with ammonium carbonate in the presence of zirconia balls, at room temperature followed by calcination of the doped titania particles at 400° C., in order to remove residual ammonium carbonate completely. The P25 titania powder as used consisted of 77 wt % anatase and 23 wt % rutile. During planetary ball milling with ammonium carbonate, anatase was gradually transformed to rutile with small quantity of brookite. The photocatalytic activity carried out under irradiation of light wavelength>510 nm, of doped titania prepared with ammonium carbonate increased at first up to 15 min and then gradually decreased. It was observed that at 15 minutes, the amount of doped nitrogen was only 0.06%; the prolonged ball milling to 180 minutes increased the amount of doping to 0.19%. However, it was observed that although the amount of doped nitrogen increased, the photocatalytic activity gradually decreased. These results suggest that the prolonged milling resulted in decreasing the photocatalytic activity due to the rutile formation, lattice distortion and powder agglomeration.
Shifu et al. in Chemical Physics Letters 413(2005) 404-409 reported synthesis of photocatalytic nitrogen doped titania by ball milling titania (100% anatase) nanoparticles (crystallite size of 30 nm) in an ammonia solution for 120 hours and air drying the powder at 110° C. in air. It was observed that with the ball milling time, the doped amount of nitrogen in the doped titania nanoparticles increased gradually which further increased photocatalytic activity. It was reported that for superior photocatalytic activity of the doped titania particles, the proper range of doped nitrogen was 0.25%, which was obtained after ball milling for 120 hours.
Yin et al. in Solid State Communications 137(2006), 132-137 reported synthesis of photocatalytic nitrogen doped TiO2 by planetary ball milling of P25 titania powder with ammonium carbonate or urea in the presence of zirconia balls, at room temperature. The P25 titania powder consisted of 77 wt % anatase and 23 wt % rutile. The ball milled samples were washed with water and dried at 50° C. for 1 day instead of calcination at 400° C. During planetary ball milling with ammonium carbonate, anatase was gradually transformed to rutile with small quantity of brookite.
Yuchao Tang et al. in Applied Mechanics and Materials Vols 71-78 (2011), pp 748-754 reported synthesis of photocatalytic N doped TiO2 by planetary ball milling of raw amorphous titania powder with nitrogen compound like ammonium fluoride (NH4F) in presence of water, for 180 min. The wet powder was dried at a temperature of 105° C. in air for 5 hours and then calcined at 400° C. for 2 hours. Use of other nitrogen compounds such (NH4)2CO3, NH4F, NH4HCO3, NH4COOCH3, CH4N2O were also reported, with highest visible absorption of doped titania, when milled with NH4F and weakest with NH4HCO3. Photocatalytic degradation was carried out under ultraviolet light and sunlight. Characterization of the catalysts demonstrated that the nitrogen doped TiO2 could improve visible light adsorption efficiency; however TiO2 surface structure was destroyed by ball milling resulting in a reduced photocatalytic activity.
Aysin et al. (in Brno, Czech Republic, EU, 21.-23.9.2011) reported photocatalytic efficiency of the silver loaded nano-sized photocatalytic titania powder prepared by ball milling photocatalytic titania powder (anatase), with 0.1 M silver nitrate solution and 1% sodium carbonate solution. Photocatalytic performance was evaluated under UV light illumination. It was observed that though the silver loading enhances the photocatalytic activity, as amount of silver loading increased, the photocatalytic activity of doped titana powder decreased.
Ramida Rattanakam et al. in Res Chem Intermed (2009) 35: 263-269 reported preparation of N doped TiO2 by a mechanochemical method using high-speed ball milling of P25 TiO2 with nitrogen source such as ammonia solution, hexamine and urea. The photocatalytic activity of the N doped TiO2 was evaluated under visible-light/sunlight irradiation. The results indicated a slight anatase to rutile phase transformation during the mechanochemical process. It was observed that although the N doped titania photocatalysts were capable of absorbing visible light of wavelength up to 545 nm, the photocatalytic activity of the doped titania particles was not improved as compared to the starting P25.
In the prior art literature, the photocatalytic doped titania nanoparticles prepared by conventional ball milling process showed phase transformation of anatase to rutile form. Also during the doping process, residual by-products were adsorbed on the surface, affecting the activity of titania nanoparticles. To remove the undesired products on the surface of the titania nanoparticles, the nanoparticles were subjected to high temperature treatment/calcination which often lead to particle agglomeration, sintering and phase change of doped titania system. Further most of the nitrogen doped titania photocatalyst prepared by mechanical milling showed high photocatalytic activity only in presence of UV light or artificial visible light at high intensity. Under sunlight, most of them exhibited weak conversion efficiency.
Thus, in order to obviate the drawbacks associated with the prior art, there is felt a need to synthesize doped titania nanoparticles with improved photocatalytic activity under sunlight irradiation.